Migrate from Commons Lang 2 to Commons Lang 3

[timja]

Migrate from deprecated/EOL Commons Lang 2 to Commons Lang 3.

Part of the effort to remove Commons Lang 2 from Jenkins core — jenkinsci/jenkins#16404,
jenkinsci/jenkins#26105. Commons Lang 2 is EOL and carries an unfixed advisory
(GHSA-j288-q9x7-2f5v).

Moved StringUtils and builder.ReflectionToStringBuilder to org.apache.commons.lang3 across
11 files, and added the commons-lang3-api library plugin (versionless — the version comes from the
Jenkins plugin BOM).

This includes six fully-qualified references written inline
(org.apache.commons.lang.StringUtils.isBlank(...) and friends) that have no import line. Those are
invisible to the ban-commons-lang-2 enforcer rule, so an import-only rename would have left them in
place and still built green.

Testing done

mvn -B -ntp clean verify passes locally on Java 21 / macOS.
The ban-commons-lang-2 enforcer rule was left disabled because it needs parent POM
6.2116.v7501b_67dc517 or newer; bumping the parent from 5.18 was out of scope here.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

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🤖 This pull request was generated with AI assistance (Claude Code) as part of a bulk migration
across Jenkins plugins. If anything here looks wrong, please comment on this PR or contact
@parameter.